68
8272E–AVR–04/2013
ATmega164A/PA/324A/PA/644A/PA/1284/P
Bits 5:0 – ISC21, ISC20 – ISC00, ISC00: External Interrupt 2 - 0 Sense Control Bits
The External Interrupts 2 - 0 are activated by the external pins INT2:0 if the SREG I-flag and the
corresponding interrupt mask in the EIMSK is set. The level and edges on the external pins that
activate the interrupts are defined in
Table 13-1. Edges on INT2:INT0 are registered asynchro-
generate an interrupt. If low level interrupt is selected, the low level must be held until the com-
pletion of the currently executing instruction to generate an interrupt. If enabled, a level triggered
interrupt will generate an interrupt request as long as the pin is held low. When changing the
ISCn bit, an interrupt can occur. Therefore, it is recommended to first disable INTn by clearing its
Interrupt Enable bit in the EIMSK Register. Then, the ISCn bit can be changed. Finally, the INTn
interrupt flag should be cleared by writing a logical one to its Interrupt Flag bit (INTFn) in the
EIFR Register before the interrupt is re-enabled.
Note:
1. n = 2, 1 or 0.
When changing the ISCn1/ISCn0 bits, the interrupt must be disabled by clearing its Interrupt
Enable bit in the EIMSK Register. Otherwise an interrupt can occur when the bits are changed.
13.2.2
EIMSK – External Interrupt Mask Register
Bits 2:0 – INT2:0: External Interrupt Request 2 - 0 Enable
When an INT2:0 bit is written to one and the I-bit in the Status Register (SREG) is set (one), the
corresponding external pin interrupt is enabled. The Interrupt Sense Control bits in the External
Interrupt Control Register, EICRA, defines whether the external interrupt is activated on rising or
falling edge or level sensed. Activity on any of these pins will trigger an interrupt request even if
the pin is enabled as an output. This provides a way of generating a software interrupt.
13.2.3
EIFR –External Interrupt Flag Register
Bits 2:0 – INTF2:0: External Interrupt Flags 2 - 0
When an edge or logic change on the INT2:0 pin triggers an interrupt request, INTF2:0 becomes
set (one). If the I-bit in SREG and the corresponding interrupt enable bit, INT2:0 in EIMSK, are
set (one), the MCU will jump to the interrupt vector. The flag is cleared when the interrupt routine
Table 13-1.
Interrupt Sense Control
ISCn1
ISCn0
Description
0
The low level of INTn generates an interrupt request
0
1
Any edge of INTn generates asynchronously an interrupt request
1
0
The falling edge of INTn generates asynchronously an interrupt request
1
The rising edge of INTn generates asynchronously an interrupt request
Bit
76543210
–
INT2
INT1
IINT0
EIMSK
Read/Write
RRRRR
R/W
Initial Value
00000000
Bit
76543210
–
INTF2
INTF1
IINTF0
EIFR
Read/Write
R/W
RRRR
R/W
Initial Value
00000000